Materials Science Forum Vol. 879

Abstract: Metastable austenitic stainless steels transform to the deformation-induced martensite by cold working. Especially, metastable stainless steel with high nitrogen content has high age-hardening property after aging treatment. In this work, effect of nitrogen on age-hardening of metastable austenitic stainless steel (SUS304: 0.04% N, type-SUS201: 0.18% N) after cold drawing was investigated, and age-hardening mechanism was elucidated. Strength after cold drawing of SUS201 containing high N is higher than that of SUS304, and the age-hardening of SUS201 is significantly higher than that of SUS304 at the aging temperature of 200 ~ 500°C. It is suggested that strengthening mechanism of SUS201 is caused by aging products of N, because exothermal reaction in SUS201 is clearly recognized at low aging temperature by DSC analysis.

Abstract: Computational density functional theory (DFT) model of the adsorption of chlorine atoms onto the perfect Al (111) surface has been performed. The structural and electronic properties of chlorine atoms adsorbed on the surface are investigated within a supercell approach for chlorine coverages of 0.25, 0.33, 0.5 and 1 ML respectively. It is found that the adsorbates prefer on-top sites over bridge, hcp and fcc sites in low coverage while fcc sites in high coverage, and the binding energy decrease with increase of coverage due to the interactions of chlorine atoms. The discussion of geometrical and electronic analysis by plotting differential charge density distribution and projected density of states (PDOS) are presented.

Abstract: In this work, the initiation and propagation behaviors of naturally initiated small crack in a polycrystalline Ni-base superalloy under the several kinds of thermos-mechanical fatigue (TMF) conditions. The effect of phase angle between temperature and strain cycles in TMF loading on the initiation and propagation behavior of small crack was discussed. The experimental results revealed that the initiation and propagation morphologies of fatigue small crack were affected on the phase angle; the crack initiation mode was changed from the intergranular mode to the transgranular mode with the increasing of the phase angle in this study. On the other hand, when the crack growth rates of naturally initiated small crack are correlated with fatigue J-integral range, the effect of the phase angle on the small crack growth rates became negligible even if the crack propagation mode depended on it.

Abstract: In the present work, the crystal structure, microstructure and martensitic transformation path in Ni-Mn-In alloys were systematically studied. Results show that the austenite has a highly ordered cubic L2₁ structure. The martensite phase possesses a 6M incommensurate monoclinic modulated structure. The microstructure of martensite is in plate shape and self-organized in colonies. The maximum of 6 distinct martensite colonies and 24 kinds of variants in one parent grain are observed. Both of K-S and Pitsch orientation relations are found to be appropriate to describe the lattice correspondence between the parent and product phase. However, the transformation path related to Pitsch relation should be the real one that governs the transformation process in Ni-Mn-In alloys. With the determined martensitic transformation path, the formation mechanism of the microstructure of martensite phase is revealed. The 6 distinct martensite colonies are respectively generated by the six (110) planes of the cubic austenite phase during martensitic transformation. Each (110) plane transforms into four twin-related variants by changing the directions of the transformation plane and direction.

Abstract: The effect of the grain size on the high-temperature oxidation resistance of unalloyed titanium was experimentally investigated using titanium samples with two different grain sizes of 219 μm and 118 μm. The weight gain during oxidation and the penetration depth of oxygen from a metal surface were larger in the small-grain-size sample compared with the large-grain-size sample. In addition, oxygen diffusion was faster in the substrate of the small-grain-size sample. These results were attributed to the grain-boundary diffusion of oxygen. A steep change in the oxygen concentration was observed at a grain boundary. Our simulation results suggested that slower oxygen diffusion into the inner grain from the surface through the grain boundary with high diffusivity can cause the observed steep change in the oxygen concentration.

Abstract: Grain boundary character distributions (GBCD) of OFHC copper equal-channel angular pressing (ECAP) deformed and then annealed were analyzed by electron back scatter diffraction (EBSD). The experimental results showed that a combination of ECAP deformation and annealing treatments could significantly increase the fraction of low-Σ coincidence site lattice (CSL) boundaries (Σ≤29) and effectively interrupt the connectivity of random boundaries network in OFHC copper. An increase of low-Σ CSL boundaries from 45.27 to 71.06% was observed in as-received material after one pass ECAP strain followed by annealing at 350 °C for 48 h. The connectivity of random boundaries network was interrupted by high fraction of low-Σ CSL boundaries.

Abstract: In the current investigation, the M2 high speed steel (HSS) sample was produced by laser direct metal deposition (DMD) with a rapid manufacturing (AM) process. The overall microstructure analysed by light optical microscopy (LOM) was a gradual transition from bottom zone to top zone due to the continuous decrease of the cooling rate. The observed microstructure from SEM and XRD was consisted of a cellular or dendritic structure of ferrite, martensite, retained austenite and fine carbides. Annealing at 860 ^oC led to spheroidization of carbides. The carbides were examined by XRD to be M₆C and MC. With prolonging the annealing holding time, the more homogenous microstructure could be acquired. These studies demonstrated that annealing can improve microstructures of M2 HSS produced by DMD.

Abstract: Mg-Zn-Y alloys with long-period stacking ordered (LPSO) phases have superior strength at elevated temperatures. We studied plastic deformation and creep behavior of a Mg₉₇Zn₁Y₂ (at.%) alloy. Deformation kinking of the LPSO phase plays an important role in strengthening the alloy during compression at elevated temperatures. Growth stacking faults with Zn/Y segregation can act as obstacles to non-basal slip and deformation twinning in Mg matrix. The tensile creep strain was only about 0.01% under a tensile stress of 70MPa for 100h at 200 °C, demonstrating excellent creep resistance of this alloy. Generation and motion of basal dislocations led to bending of LPSO phase during tensile creep of the Mg₉₇Zn₁Y₂ (at.%) alloy. Plastic deformation in Mg grains was mostly achieved through basal slip during creep at temperatures below 200 °C, while non-basal slip through the generation and motion of “a + c” dislocations was activated with increasing the temperature to 200 °C and above. Dissociation of dislocations and Suzuki segregation on basal planes occurred widely in Mg matrix, which hindered dislocation motion and thus played an important role in preventing Mg grains from softening during deformation at elevated temperatures. In addition, Cottrell atmospheres were observed along dislocations in plastically deformed LPSO phase, impeding motion of dislocations. The superior strength and creep resistance of the Mg₉₇Zn₁Y₂ (at.%) alloy at elevated temperatures are thus associated with the LPSO phase, stacking faults in Mg grains, formation of Cottrell atmospheres in LPSO and occurrence of Suzuki segregation in Mg.

Abstract: Deformation behavior of Fe-15Al-18Co-3Ti (at.%) single crystals containing the Co₂AlTi precipitates was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Co₂AlTi phase with the L2₁ structure was precipitated in the bcc matrix. The L2₁ phase showed a cuboidal shape with a misfit strain of 0.59%. It is also noted that large amount of Fe substituted for Co in the Co₂AlTi precipitates. The FC single crystals exhibited high yield stress above 600 MPa up to 823 K while further increase in temperature resulted in a decrease in yield stress. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L2₁ precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L2₁ precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 823 K. The crystals with the fine L2₁ precipitates demonstrated high yield stress above 1400 MPa at room temperature. Paired 1/2<111> dislocations cut the fine L2₁ precipitates, which led to high strength. The dependence of the yield stress on the precipitate size was also discussed.

Abstract: To create a high reliability solder joint using IMCs dispersed in the joint, the joints with four types of lead-free solder were investigated. The joint with Sn-3.0Ag-0.7Cu-5.0In (mass%) has high die shear force compared to other joints investigated, and the joint with the Ni-electroplated Cu bonded at 300 ^oC for 30 min showed the maximum die shear force due to formation of a large number of fine IMCs. In the joint with Sn-0.7Cu-0.05Ni (mass%), uniform dispersion of a large number of IMCs was achieved, although the die shear force of the joint is lower than that of the joint with Sn-3.0Ag-0.7Cu-5.0In. In the joint with Sn-5.0Sb (mass%), a solder area was remained in the center of the joint although a large number of columnar IMCs form at the joint interface. The die shear force of the joint with Sn-5.0Sb increased with increasing the bonding time due to formation and growth of IMCs. In the joint with Sn-3.0Ag-0.5Cu (mass%), IMCs formed at the joint interface and did not disperse in the entire joint.